题目内容

In slow speed two-stroke engines,（）the fresh air entering begins when and stops as（）

A. the inlet port is opened by the up coming piston； the port is closed by the down going
B. the inlet valve is opened by the down going piston；the inlet valve is closed
C. the inlet port is opened by the down going piston；the port is closed by the up coming
D. the inlet valve is opened

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